1. Field of the Invention
The present invention relates to transmissions and straddle-type vehicles including the transmissions.
2. Description of the Related Art
A conventionally known transmission includes a dog clutch type transmission mechanism, a clutch actuator that causes a clutch to be disengaged and engaged, and a shift actuator that drives the transmission mechanism. The dog clutch type transmission mechanism includes a plurality of gears provided on a main shaft and a drive shaft. These gears include a movable gear that is axially movable and a fixed gear that is axially immovable. Concave or convex portions, each called a “dog portion”, are provided on lateral surfaces of the movable and fixed gears. The movable gear is disposed between the fixed gears, and the dog portion of the movable gear and the dog portion of the fixed gear are provided so as to be engageable with each other. A transmission gear ratio is changed by changing a combination of the movable and fixed gears engaged with each other.
The above-described transmission performs a speed change as follows. The clutch actuator temporarily disengages the clutch, and then engages the clutch again. The shift actuator moves the movable gear axially. In this transmission, the movable gear is disposed between first and second fixed gears, and the shift actuator moves the movable gear from the first fixed gear toward the second fixed gear. The shift actuator starts movement of the movable gear after disengagement of the clutch has been started. Upon driving of the shift actuator, the movable gear is moved away from the first fixed gear, and the dog portion of the movable gear and the dog portion of the first fixed gear are disengaged from each other. This disengagement will be referred to as “dog disengagement”. Subsequently, the movable gear moves toward the second fixed gear, and comes into contact with the second fixed gear. At this point, the dog portion of the movable gear and the dog portion of the second fixed gear are engaged with each other. This engagement will be referred to as “dog engagement”. After the dog portion of the movable gear and the dog portion of the second fixed gear have been engaged with each other, the clutch enters an engaged state.
Prior to a speed change, the movable and fixed gears are rotated together, and a frictional force is produced between the dog portion of the movable gear and the dog portion of the first fixed gear. In order to effect dog disengagement, the movable gear has to be moved with a force greater than the frictional force. When the frictional force is great, dog disengagement cannot be effected easily, thus taking a long time to complete dog disengagement. Therefore, a speed change time is prolonged. The frictional force tends to increase as an engine torque is increased. With the aim of facilitating dog disengagement, JP 2004-11774 A discloses a technique in which an engine torque is temporarily decreased by performing ignition cutoff, and then movement of a movable gear is started. The speed change time can be reduced by facilitating dog disengagement.
It is conceivable that the movable gear may be moved quickly in order to reduce the speed change time. However, when the movable gear is moved quickly, the movable gear strongly collides with the second fixed gear even though a time required for dog disengagement is reduced. In this case, a phenomenon that the dog portion of the movable gear and the dog portion of the second fixed gear hit against each other without engaging with each other, i.e., “dog hitting”, is likely to occur. JP 2012-225436 A discloses a technique in which a movable gear is moved quickly at the time of dog disengagement, but the movable gear is braked immediately before dog engagement.
The techniques disclosed in JP 2004-11774 A and JP 2012-225436 A are both used in order to reduce the speed change time. However, when movement of the movable gear is started after clutch disengagement has been started and then the engine torque has been decreased as disclosed in JP 2004-11774 A, a driver may feel that the speed change time is long even if a time required for dog disengagement is short. When a time period between start of clutch disengagement and start of movement of the movable gear is short but a time period between the start of movement of the movable gear and end of the movement is long as disclosed in JP 2012-225436 A, a driver may feel that the speed change time is long. Hence, the above-described conventional techniques cannot effectively and reliably reduce the speed change time perceived by a driver.